1. Field of the Invention
The present invention relates to an optical switch used in long-distance, large capacity optical fiber communication and the like for switching a controlled light using a control light.
2. Description of Related Art
To realize large capacity optical fiber communication using limited communication line resources effectively, means for increasing the number of channels capable of transmission and reception and means for increasing the communication speed must be provided.
Multiplex communication methods such as time division multiplexing (TDM) are under investigation as such means for increasing the number of channels. TDM is a communication method in which multiplexing/demultiplexing means are used to time-multiplex a plurality of channels (tributary channels), transmit the time-multiplexed channels as a time-division multiplex signal, and then divide the time division multiplex signal into the individual channels (tributary channels) on the reception side by means of a gate signal generated from a clock signal in order to extract and receive the information in the individual channels.
To increase the communication speed of the TDM described above, it is desirable that all of the multiplexing/demultiplexing means be realized by optical means. In other words, it is desirable to realize an optical switch which is capable of executing a switching operation for transmitting or blocking an optical pulse constituting an optical pulse signal, which is a controlled light, using only an optical control signal serving as a control light, rather than electrical means.
The optical Kerr effect that occurs in optical fiber is a phenomenon by which the refractive index of an optical fiber is altered when light of high intensity propagates through the optical fiber. The reaction rate of the optical Kerr effect is several femto-seconds (fs). In other words, if an optical switch is constituted using the optical Kerr effect, an optical switch that is capable of switching optical pulse signals at several hundred Gbit/s or more can be realized. By comparison, a conventional switch, in which an optical pulse signal is first converted into an electric pulse signal serving as an electrical signal, whereupon the electric pulse signal is switched by an electronic device and then returned to an optical pulse signal, is capable of switching an optical pulse signal at a maximum bit rate of approximately 40 Gbit/s.
An optical switch which utilizes the optical Kerr effect generated in a polarization-maintaining single-mode fiber is under investigation as an optical switch utilizing the optical Kerr effect (see “Ultrafast optical multi/demultiplexer utilising optical Kerr effect in polarisation-maintaining single-mode fibres”, T. Morioka, M. Saruwatari, and A. Takada, Electronic Letters, vol. 23, No. 9, pp. 453–454, April 1987, for example).
The optical switch utilizing the optical Kerr effect disclosed in “Ultrafast optical multi/demultiplexer utilising optical Kerr effect in polarisation-maintaining single-mode fibres”, T. Morioka, M. Saruwatari, and A. Takada, Electronic Letters, vol. 23, No. 9, pp. 453–454, April 1987 employs polarization-maintaining single-mode fiber (also referred to hereafter simply as “optical fiber”) as optical fiber for generating the optical Kerr effect. Polarization-maintaining single-mode fiber is constituted such that the effective index in relation to light guided therethrough differs between an optical axis direction known as the slow axis, which is set on a perpendicular plane to the propagation direction of light through the fiber (also referred to as the “optical axis direction of the optical fiber” hereafter), and an optical axis direction known as the fast axis, which is orthogonal to the slow axis.
In other words, a stress-applying portion having a higher refractive index than the refractive index of the cladding, is placed in the vicinity of the core of the optical fiber such that the effective index in relation to light in which the vibration direction of the electric field vector of the light is parallel to the slow axis direction is higher than the effective index in relation to light in which the vibration direction of the electric field vector of the light is parallel to the fast axis direction. As a result of this asymmetry in the effective index, light input into the polarization-maintaining single-mode fiber is propagated with a maintained polarization plane. Hereafter, the vibration plane of the electric field vector of linearly polarized light will also be referred to as the polarization plane.
The optical fiber used in the optical switch disclosed in “Ultrafast optical multi/demultiplexer utilising optical Kerr effect in polarisation-maintaining single-mode fibres”, T. Morioka, M. Saruwatari, and A. Takada, Electronic Letters, vol. 23, No. 9, pp. 453–454, 1987 has a plane formed by fusing two polarization-maintaining single-mode fibers such that the optical axes thereof are orthogonal, and is constituted such that the birefringence of the polarization-maintaining single-mode fiber can be canceled out. The optical switch is input with a linearly polarized control light having a parallel polarization plane to the optical axis of the polarization-maintaining single-mode fiber, and a linearly polarized signal light having a polarization plane inclined by 45 degrees from the optical axis of the polarization-maintaining single-mode fiber.
When an optical pulse constituting a signal light and an optical pulse constituting a control light are not input into the optical switch in synchronization, the signal light optical pulse is output in the same linearly polarized state as it was when input into the optical switch. On the other hand, when a control light optical pulse and a signal light optical pulse are input in synchronization, the control light optical pulse causes the optical Kerr effect to be generated on the polarization component of the signal light optical pulse that is parallel to the polarization direction of the control light optical pulse. In other words, the optical Kerr effect produces a phase shift in the signal light optical pulse due to a mutual phase modulation effect generated between the signal light optical pulse and control light optical pulse.
This phase shift φ is obtained using the following equation (1).φ=2γPL  (1)
Here, P(W) is the power of the control light, and L(km) is the length of the optical fiber constituting an optical fiber loop. γ(W−1km−1) is a nonlinear optical constant based on the optical Kerr effect. The value of γ(W−1km−1) in relation to normal optical fiber is approximately 1 to 2 W−1km−1, but special optical fiber known as highly nonlinear optical fiber, having a reduced effective sectional area such that the value of γ(W−1km−1) is approximately several ten to several hundred W−1km−1, is also under development.
When the phase shift amount φ is equal to π, the polarization direction of the signal light optical pulse rotates 90 degrees from the time of input into the optical switch. In other words, the polarization direction of the signal light optical pulse is −45 degrees from the optical axis of the optical fiber. By disposing an analyzer at the output side of the optical switch, the signal light optical pulse can be transmitted or blocked by means of the control light. More specifically, by setting the optical axis direction of the analyzer such that the signal light optical pulse is transmitted when the polarization direction thereof is rotated 90 degrees from the time of input into the optical switch and blocked when the polarization direction thereof is identical to the polarization direction at the time of input, only optical pulses whose polarization direction has been rotated by the control light can be transmitted through the optical switch, and thus the signal light optical pulse can be switched by the control light.
To ensure that the switching operation described above is performed reliably, a prerequisite of the optical switch disclosed in “Ultrafast optical multi/demultiplexer utilising optical Kerr effect in polarisation-maintaining single-mode fibres”, T. Morioka, M. Saruwatari, and A. Takada, Electronic Letters, vol. 23, No. 9, pp. 453–454, 1987 is that the respective polarization states of the signal light optical pulse and control light optical pulse are maintained during propagation through the polarization-maintaining single-mode fiber. In this optical switch, the polarization states of the optical pulses are maintained substantially by providing a constitution (also referred to as “fused portion”) in which polarization-maintaining single-mode fibers are fused so as to have orthogonal optical axes in an intermediate position in the optical axis direction of the polarization-maintaining single-mode fiber.
To describe more specifically the manner in which the polarization state of the optical pulses is substantially maintained, first a first stage polarization-maintaining single-mode fiber extending from the entrance terminal of the polarization-maintaining single-mode fiber to the fused portion, and a second stage polarization-maintaining single-mode fiber extending from the fused portion to the exit terminal of the polarization-maintaining single-mode fiber, are provided. Then, a linearly polarized light having a polarization plane that is inclined 45 degrees from the optical axis of the first stage polarization-maintaining single-mode fiber is input. The components of the input light that are parallel to the fast axis and slow axis of the first stage polarization-maintaining single-mode fiber at this time are defined as an S component and a P component respectively. The phase difference that occurs between the S component and P component of the input light in the first stage polarization-maintaining single-mode fiber is set as φ.
Thus the polarization-maintaining single-mode fiber constituting this optical switch is designed such that the phase difference which occurs between the S component and P component of the input light in the second stage polarization-maintaining single-mode fiber becomes −φ. In other words, the fused portion described above is provided on the path taken by the signal light from the input terminal of the first stage polarization-maintaining single-mode fiber, which is the input terminal of the optical switch, to the analyzer, in a position in which the optical path length (a value obtained by multiplying the refractive index by the geometrical length) when the signal light is input onto the path as TM polarization matches the optical path length when the signal light is input as TE polarization.
Hence, to realize the operation described above, the first stage polarization-maintaining single-mode fiber and second stage polarization-maintaining single-mode fiber must have an identical constitution, and the length of the first stage polarization-maintaining single-mode fiber must be set equally to the length of the second stage polarization-maintaining single-mode fiber.
In PANDA (polarization-maintaining AND absorption-reducing) optical fiber, which is used widely as polarization-maintaining single-mode fiber, the difference between the effective index when the vibration direction of the electric field vector of the guided light is parallel to the fast axis and the effective index when the vibration direction of the electric field vector of the guided light is parallel to the slow axis is approximately 3×10−4. Hence, if the wavelength of the guided optical pulse is 1.5 μm, for example, a phase difference of 2π is generated between the component in which the vibration direction of the electric field vector is parallel to the fast axis and the component in which the vibration direction of the electric field vector is parallel to the slow axis by propagating an optical pulse approximately 5 mm through the polarization-maintaining single-mode fiber.
In other words, if the difference between the length of the first stage polarization-maintaining single-mode fiber and the length of the second stage polarization-maintaining single-mode fiber cannot be set sufficiently below 5 mm, the polarization state of the optical pulse propagating through the polarization-maintaining single-mode fiber cannot be substantially maintained.
The length of the optical fiber used in this type of optical switch is typically between several tens of meters and several hundred kilometers, and it is therefore extremely difficult to set the entire length of such an optical fiber with an accuracy of millimeters or less. Moreover, if the difference between the length of the first stage polarization-maintaining single-mode fiber and the length of the second stage polarization-maintaining single-mode fiber is not set to a sufficiently small value, it becomes impossible to maintain the characteristics of the optical switch in the face of variation in the ambient temperature of the optical switch and the wavelength of the signal light or control light.
Moreover, the fast axis (or slow axis) direction of a polarization-maintaining single-mode fiber such as a commercially available PANDA optical fiber is not always completely unchanging in the length direction thereof. Therefore, even when an input light input into the polarization-maintaining single-mode fiber is a linearly polarized wave having a polarization plane that is parallel to the fast axis (or slow axis) of the polarization-maintaining single-mode fiber, the output light that is output from the polarization-maintaining single-mode fiber comprises a polarization component that is orthogonal to the polarization direction of the input light. This component having an orthogonal polarization direction to the polarization direction of the input light is known as polarization cross talk.
It is known that in commercially available PANDA optical fiber having an average polarization-maintaining capability, this polarization cross talk increases dramatically when the length of the PANDA optical fiber increases beyond several tens of meters (see “Polarization Maintaining Fiber”, Arai, Saito, Koyama, Nakamura, Yokomizo, Aiso, Furukawa Electric Information No. 109, pp. 5–10, January 2002, for example).
As described above, an optical switch utilizing the optical Kerr effect is typically constituted with polarization-maintaining single-mode fiber of several tens of meters or more, and therefore considerable attention must be paid to polarization cross talk during design. An optical pulse propagating through the polarization-maintaining single-mode fiber constituting the optical switch comprises polarization components in both the fast axis and slow axis directions. Hence, if polarization cross talk occurs, the polarization cross talk interferes with the original polarization direction components of the signal light optical pulse such that the polarization state of the signal light optical pulse differs from a case in which polarization cross talk does not exist. The effect of this polarization cross talk on the signal light optical pulse also varies according to variation in the wavelength of the signal light optical pulse, the ambient temperature of the polarization-maintaining single-mode fiber, and soon. In other words, polarization cross talk causes variation in the operating characteristics of the optical switch, which leads to instability in the switching operation.
It is therefore an object of the present invention to provide an optical switch capable of realizing a stable operation, whose the operating characteristics are not altered, and which is not affected by polarization cross talk, even when the wavelength of a signal light serving as a controlled light and the ambient temperature of the optical switch vary.